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IN DEEP GALAXY SURVEYS, ASTRONOMERS GET A BOOST – FROM GRAVITY

SEATTLE – Astronomers who survey galaxies in the distant universe are getting some unexpected help from gravity, according to a new study.

In a presentation at the American Astronomical Society meeting this week and a related paper in the current issue of the journal Nature, researchers say that as many as 20 percent of the most distant galaxies currently detected appear brighter than they actually are, because of an effect called “strong gravitational lensing.”

The discovery could change astronomers’ notions of how galaxies formed in the early universe.

“On one hand, lensing is good for us in that it enables us to detect galaxies that would otherwise be invisible, but on the other hand we will need to correct our surveys to obtain accurate tallies,” Yan said.

“On one hand, lensing is good for us in that it enables us to detect galaxies that would otherwise be invisible, but on the other hand we will need to correct our surveys to obtain accurate tallies,” Yan said.

He explained how strong gravitational lensing works. From our view on Earth, if a faraway galaxy and a nearby galaxy line up on the sky, the gravity of the nearby galaxy bends the light from the faraway galaxy, as if the nearer galaxy were a magnifying glass, or lens.

Einstein predicted decades ago that gravity could bend light, and astronomers have since proven him right. In fact, modern astronomers exploit the effect to find distant objects that would otherwise be invisible, such as planets orbiting other stars.

And in this case, a statistical analysis revealed that gravitational lensing is brightening faraway galaxies that would otherwise be too faint to see.

Now that astronomers are aware of the effect, they can use it to their advantage.

“We just need to be aware that we are looking through ‘lenses,’” Yan said. “Take a real magnifying glass as an example: through the lens you can see more details of an object – which is a good thing – but you should be aware that all those details are not actually the sizes that you see, because you're looking through a magnifying lens. The same is true when we’re looking at galaxies.”

“We predict that many galaxies in the most remote universe will only ever be visible to us because they are magnified in this way,” he added.

The survey looks back in time 13 billion years, to when the universe was less than one billion years old. Astronomers want to know how many galaxies were bright or faint when the universe was still in that infant stage. So any magnification of those galaxies will interfere with astronomers’ ability to judge.

Even through the eyes of Hubble, these faraway galaxies look very small, so it’s hard to tell which ones have been magnified. Yet tallies from the HUDF survey are critical to scientists’ understanding of how galaxies formed and evolved.

“Although we do not yet have an instrument to directly detect a lensing ‘signature’ to unambiguously support our prediction, we now have some indirect, tentative evidence that the number of lensed galaxies could be high as we look into the early universe,” Yan said. "The apparent association of very distant galaxies to galaxies in the foreground is the key."

Sometimes, the gravitational lensing distorts a galaxy’s appearance, or alters its brightness. Other times, the lens splits the light from the faraway galaxy so that two or more galaxies will form around the lens, when there is really only one.

In fact, Yan and his colleagues began this work in order to understand why so many of the faraway galaxies they observed in HUDF survey images appear to be located near the line of sight to galaxies in the foreground.

Through a statistical analysis, they determined that strong gravitational lensing is the most likely explanation.

Yan stressed that the 20-percent estimate is an initial one, and could change in the future.

“We want to make it clear that the size of the effect depends on a number of uncertain factors. If, for example, very distant galaxies are much fainter than their nearby counterparts but much more numerous, the majority of such distant galaxies that we will detect in the foreseeable future could be lensed ones,” he said.

According Yan and his colleagues, the impact of gravitational lensing on galaxy surveys will be even higher in future studies.